If you are biking on a flat road where there is relatively no danger of getting hurt, it doesn't matter if you are wearing a helmet or not. If you are doing 30+ mph descents down hills, it makes a lot of sense to be wearing a good helmet.

A helmet saved my life and that will always make me wear one. I flipped off the front of the bike and landed headfirst on a really big rock and broke the helmet clean in half; I don't want to think about that being my skull.

certain helmets are actually dangerous if you're riding on the street. the ones with big fins coming off the back for aerodynamics can potentially bend your neck to the point of breaking it if you fall on it the wrong way.